In Computed Radiography (CR), a phosphor sheet is exposed to x-ray or other short-wavelength ionizing radiation and stores a latent image that is read out by a scanning device. Within the scanning device, read-out is effected by illuminating the sheet, one spot or pixel (picture element) at a time, with a stimulating beam of a first relatively long-wavelength light, such as with red laser light. When it receives the stimulating beam, the illuminated spot on the phosphor sheet emits radiation at a second, shorter wavelength, typically in the blue region. The amount of radiation that is emitted upon stimulation is proportional to the amount of energy stored as a result of x-ray exposure. An optical collector, including a Photo-Multiplier Tube (PMT) or other type of photodetector device, obtains image content by sensing the relative amount of stimulated light of this second wavelength, one pixel at a time, in a scanning sequence that moves across the surface of the phosphor sheet.
One class of CR equipment employs a flexible phosphor sheet as its storage medium. The scanning apparatus for this type of medium loads the flexible storage sheet with its back against the inner, concave surface of a cylindrical drum. In this type of internal drum scanner, the stimulating laser beam spins radially as it is moved linearly in an axial direction, tracing out a helical scan pattern; a light collector travels along the axis with the spinning beam, recording the stimulated light. This arrangement allows the scanner to be relatively compact and has optical advantages for obtaining the stored image data in a uniform manner across the surface of the flexible phosphor sheet. One exemplary scanner of this type is described in U.S. Pat. No. 6,624,438 entitled “Scanning Apparatus” to Koren.
While conventional CR scanning methods have achieved some level of success, it has proven difficult to improve image quality beyond a certain point. One innate difficulty relates to the relative inefficiency of the phosphor material itself. Due to low efficiency levels, the photomultiplier tube (PMT) or other detector must be very sensitive in order to capture the image signal and is thus highly susceptible to noise. Compromises made to increase the signal strength or sensitivity work counter to the need for keeping noise levels low. Thus, increasing the signal-to-noise ratio appears to be an elusive goal.
The internal drum scanner is designed to eliminate ambient noise from other light sources. However, because of its cylindrical geometry, this type of scanner can be subject to a false signal, termed “flare”. Flare results because a significant portion of the stimulating beam reflects from the surface of the phosphor sheet instead of being absorbed. This stray light, traveling inside the drum, can impinge on other portions of the phosphor sheet, inadvertently causing emission from areas other than the stimulated spot or pixel. Flare degrades collection efficiency in two ways: exciting a false signal by premature emission from areas of the surface outside the scanned spot and draining stored energy due to such emission in portions of the phosphor that have not yet been scanned.
Conventional approaches have been applied in attempts to reduce the occurrence and effects of flare. In the scanner described in the Koren '438 disclosure, a filter is provided against the input aperture of the sensing photomultiplier tube. This filter reduces flare by transmitting the second stimulated frequency and absorbing the first stimulation frequency. Other designs provide a narrow slit through which the stimulating beam is directed, reducing the range of angles permitted for reflected stimulating light. However, this type of approach can also restrict the amount of stimulated light that is able to enter the collector from the spot being scanned. As a consequence, providing a suitable slot width generally involves a compromise that tolerates more flare than is desired and achieves less collection efficiency than is desired.
Thus there is a need for an internal drum CR scanner that reduces the likelihood and susceptibility to flare and increases the amount of signal, thereby providing improved signal-to-noise characteristics and higher quality radiographic imaging.